Terms which have been used interchangeably in industry to refer to a construction provided at the end of an insulated high voltage cable to bring about a desired potential gradient from the base conductor to the conductive sheath of the high voltage cable include termination, terminator, terminal and potential head or "pothead". The terminology presently employed in describing cable terminations is generally covered in a publication by the Institute of Electrical and Electronic Engineers (I.E.E.E.) identified as Std 48-1975 entitled "IEEE Standard Test Procedures and Requirements for High Voltage Alternating-Current Cable Terminations", issued May 9, 1975.
In preparing the end of a high voltage cable for termination, the outer conductive sheath or shield layer is normally cut back a predetermined distance from the end of an insulation layer which, in turn, is cut back from the axial conductor so as to expose an end portion thereof. The shield layer is cut back sufficiently to provide adequate creepage distance between the live conductor and the grounded shield layer. It is known that if two electrically conducting bodies, such as an axial conductor and a coaxial conductive shield layer of a high voltage cable, are separated by a distance small in comparison with the surface dimensions of the conducting bodies, the electrical field in the region between the bodies is substantially uniform and perpendicular to the surfaces of the conducting bodies. The difficulty, however, comes at the boundaries of the conducting bodies or at the boundary of one of them if its area is much less extensive than the area of the other conductor, such as when the shield layer of a high voltage conductor cable is cut back from the axial conductor and a portion of the insulation layer.
Cutting back the shield and insulation layers creates an abrupt discontinuity in the electrical characteristics of the cable and materially increases the maximum voltage gradient (volts/mil) of the insulation in the area of the shield end. The increase in voltage gradient at the shield end changes the shape of the resulting electrical field so that the stress is no longer uniform and normal to the adjacent surfaces of the conducting bodies but produces a large component of stress along the surface of the insulation in a direction parallel to the conducting surfaces. Thus, the maximum voltage gradient is shifted from a radial stress, which diminishes outwardly from the conductor, to a longitudinal stress at the end of the cable shield layer. The nature of the cable insulation is such that it more readily withstands an electrical stress in the radial direction than along its longitudinal surface so that the risk of breakdown is substantially greater in the longitudinal direction.
At the present time, almost all electrical power is generated and transmitted as AC current, the transmission of AC current resulting in minor impedance losses relative to the transmission of DC current. AC transmission does, however, result in substantial dielectric loss whereas DC transmission results in relatively minor amounts of dielectric loss. At very high voltages, i.e., above 1,000 KV, the total DC power loss through impedance and dielectric loss is less than the total power loss resulting from AC transmission, and it is contemplated that power will be increasingly transmitted by DC current in the future and effective terminations are needed for high voltage DC cable.
For terminations of relatively low voltage cables, whether AC or DC, it is sufficient to interpose an effective dielectric layer between the unshielded end of the cable and the grounded end of the shield. For high voltage AC cable, i.e., above about 100 KV, capacitive elements are commonly used to relieve the electrical stress between the high potential of the conductor and the grounded (zero potential) end of the shield. U.S. Pat. No. 4,228,318 is an example of a termination for AC-carrying cable which incorporates a stress relief cone extending from the end of the cable shield and capacitors stacked in coaxial relation along the length of an exposed insulation layer and the underlying axial conductor. If constructed of sufficient size, such terminations may be made sufficiently large to terminate cables carrying upward of 600 KV AC.
Electrical stress in DC cable terminations is not relieved by capacitor shields when the current is constant but may be relieved instead through a highly resistive path between the conductor and ground which provides a suitable gradient of electrical potential to prevent dielectric breakdown with a minimal power loss therethrough. A steady current, however, is not maintained at all times in high voltage DC transmission, and there are many situations where the potential varies greatly, e.g., during polarity reversals. When the DC potential is changed, substantial transient currents are produced in DC cable, and a DC terminator must relieve the electrical stress resulting from these transients.
U.S. Pat. No. 3,539,703 discloses a high voltage termination for AC or DC-carrying cable. A plurality of conducting members and a plurality of insulating members are alternately wrapped in overlapping relationship, providing capacitance shielding for AC applications. For DC termination, the wrapped conducting members are interconnected through their edges by a spiral of resistors which provide a highly resistive pathway between the conductor and ground. The capacitive-resistive shielding is covered with an outer layer of insulating material which covers all the projections of the resistors.
Terminations having electrical shielding of the type described in U.S. Pat. No. 3,539,703 may be effective in laboratory situations; however, they would not be useful in high voltage DC electrical installations, i.e., above about 100 KV. The spiral of resistors interconnecting the conductive elements would produce significant amounts of heat that would not be effectively dissipated in the outer layer of insulating material, resulting in eventual burn-out of resistors. The failure of any resistor within the spiral would destroy the DC stress relief. Furthermore, it is believed that the capacitive arrangement could not be utilized for AC applications above about 100 KV.